1. Technical Field
The present invention relates to a device for supplying pressure to an actuation unit of a vehicle brake system, in particular to a vehicle brake system of the ‘brake-by-wire’ type, comprising a pneumatic motor-and-pump assembly with a pump and a motor driving the pump, wherein the motor-and-pump assembly is driven by an electronic control unit depending on a pressure level or pressure difference in a pneumatic brake booster of the actuation unit, with the pressure level in a chamber or a difference in pressure between two chambers of the brake booster being detected by means of a sensor.
2. Description of the Related Art
A device of this type and a method for supplying vacuum in a vehicle is e.g. disclosed in DE 199 29 880 A1, wherein the motor-and-pump assembly comprises a vacuum pump, which is additionally activated depending on operating conditions of the driving unit.
Vehicle brake systems of the ‘brake-by-wire’ type are operable in response to the driver's request by means of an electronic control unit (ECU), i.e. irrespective of the driver, as well as partly by means of a brake pedal (mechanical fallback mode). It is disadvantageous in systems without a boosted mechanical fallback mode that the service brake function, i.e. boosted braking, is given only when the electric energy supply exists. Failure of the electric energy supply or a defect in the electric control unit will cause that e.g. in a hydraulic brake-by-wire bake system, that means an electro-hydraulic brake (EHB), only non-boosted braking is possible in conformity with legal conditions (0.3 g with 500 N pedal force). In general, the failure probability of the electronic components, in particular the electric energy supply, is greater than that of the conventional, mechanical brake components so that enhanced technical measures must be taken, in order to maintain the availability of a brake-by-wire vehicle brake system in terms of the service brake function to a similarly high degree as in a conventional vehicle brake system.
Among others, this objective, i.e. enhancing the availability of the service brake function compared to an EHB, is also aimed at by the use of a regenerative brake based on a brake booster that is actuatable irrespective of the driver. The regenerative brake is mainly used in hybrid and electric vehicles. When this brake is envisaged for use in hydraulic vehicles equipped with an electric motor and an internal combustion engine, operating conditions are encountered in which the internal combustion engine does not supply pressure to the actuation unit, and therefore an electrically driven pump becomes necessary.
In particular in purely electrical vehicles, the pressure required must be generated by way of such a pump alone. Thus, the service brake function again depends on the availability of the electrical wiring system. Frequent attempts have been made for cost reasons to execute the control of the vacuum pump by way of the ECU of the brake system, whereby the service brake function also depends on the availability of the brake control unit.